Presently, the bulk volume of the world production of photovoltaic elements comprising solar panels is based on multi-crystalline silicon wafers cut from ingots that are cast by directional solidification (DS) based on the Bridgeman or Vertical Gradient Freeze (VGF) methods in electrically heated furnaces. The crucible being employed is usually made of silica SiO2, and the furnaces have heating devices above, below and/or sideways with respect to the crucible to provide the heat for melting and control of heat extraction during the directional solidification. The process may be summarised as follows.
A crucible, open at the top, made of SiO2 is covered in its interior with a silicon nitride containing coating and filled with a silicon feedstock to a predetermined height. The crucible is then placed on the floor of a process chamber of the furnace. Next, a circumferential support structure of graphite plates is attached along the outer crucible walls to provide mechanical support at elevated temperatures when the SiO2 crucible sags. The furnace compartment is then closed, evacuated and the inert purge gas is supplied during the period when the heating elements are engaged so as to cause the melting/solidification of the silicon feedstock. When the silicon is melted, the heating is adjusted to obtain a directional solidification. An inert purge gas, usually argon, is flushed onto the surface of the silicon to protect against gaseous contamination and to provide effective removal of SiO gas for at least as long as the silicon is in the liquid phase.
One of the major challenges in these processes is to maintain a flat or slightly convex solidification front (as seen in the vertical direction from solid phase to melt) during the entire directional solidification process. This is considered important to minimise defects such as dislocations. Defects of this kind can have a deleterious effect on the suitability of the formed crystal for its purpose. For example, if the directionally solidified material is silicon which is to be processed into solar cells, defects such as dislocations can lead to deterioration in the efficiency of the ultimate solar cell.
Accordingly, as a consequence of a flat crystallization profile requirement, it is necessary to ensure that the temperature across the horizontal plane perpendicular to which the material solidifies is constant. However, given the relatively large size of production furnaces this is not easy to achieve due to non-uniform heat distribution related to the heat losses through the sides of the furnace.
In a simple, one-ingot furnace, a conventional design for a heating device disposed above the crucible usually consists of two parallel conductor rails connected by resistive heating elements which extend between and perpendicular to the rails. Heat is generated by passing current through the heating elements. While this design has benefits in terms of simplicity of manufacture and consequent cost, the uniform vertical heat flux initially produced by the heating device deteriorates at the sides of the furnace due to side cooling effects, therefore the net effect is a crystallization profile that is distorted from its intended flat shape. In this way, non-uniformity in the heat flux is detrimental to the quality of the produced crystalline material.
Moreover, these difficulties are increased when attempting to scale up the production of silicon wafers. Scaling of the production processes plays an important role in a reduction of a silicon wafer cost without compromising quality. Multi-ingot furnaces, which produce several ingots per run compared to one-ingot furnaces, represent an attractive economic solution. As a matter of fact, for a smaller ingot it is easier to control crystallization parameters and material uniformity within respective ingots, the latter results in a superior final quality of wafers. But despite obvious quality-related benefits, the production of multiple ingots of smaller size instead of one large ingot poses a major challenge with respect to the organization of correct heat fluxes in the furnace. In particular, the different positions of ingots within the furnace are not thermally equivalent, since they will be affected differently by the side cooling effects mentioned above. This leads to an inherent asymmetry in the heat flux across each ingot.
For example, consider a four-ingot furnace in which ingots are arranged in a 2×2 array. For each ingot, two sides are internal (i.e. facing another ingot) and the two other sides are external (i.e. facing the sides of the furnace). Whereas a single-ingot furnace at least retains a symmetry in the vertical heat flux around a central point of the ingot, this is no longer the case for the ingots in a multi-ingot furnace. The side heat losses may result in improper temperature gradients across each ingot and increased thermal stresses.
Another issue, which arises especially for relatively large ingots is a non-simultaneous finalisation of crystallization process. That is, the crystallization process is not completed simultaneously across the surface of an ingot. As a result, while when one part of the ingot is already fully crystallized, another part is still covered with molten material. The effect causes deviation from presumed cycle time of the production process.
Efforts have been made to improve the uniformity of the net heat flux provided by heating devices, the uniformity of action of heat extraction mechanisms, and insulation to negate factors relating to the overall design of the furnace, but there remains a need to further improve the performance of furnaces in this regard.